I created a simple application that demonstrates my understanding of your problem, please, find it attached. However, it seems that it works properly with the latest internal build (that is not released yet). Am I doing something wrong? You could also try to upgrade the assemblies in your application with the latest internal build available and see if it works, because the problem could be caused by a bug that has been already fixed. I also saw that you have a handler for the Loaded event in RadComboBox - it could contain code that causes the problem, if the upgrade and my application did not help, I would like to see what's in this handler.
the Telerik team